The present Invention relates in general to a novel method and apparatus for generating micron wavelength radiation (infrared radiation) and, more particularly, to a novel rapidly tunable quasi-free electron laser for use at micron wavelengths.
Various techniques have been proposed for generating radiation in the millimeter and submillimeter wavelength regions including miniaturized microwave tube sources, quantum mechanical sources, and stimulated scattering from relativistic electron beams. For example, in U.S. Pat. No. 3,639,774 far infrared radiation is generated by backscattering microwaves from a relativistic electron beam having a direction along a parallel magnetic field of a magnitude chosen to make the electron cyclotron frequency approximately equal to the microwave frequency. Coherent radiation at far infrared wavelengths is then produced by resonating the backscattered radiation to produce stimulated emission of the far infrared radiation. While successful in generating far infrared radiation, this technique has limited application due to its inherent low power and limited tunability.
U.S Pat. No. 3,822,410 discloses a tunable high power radiation source for use in the range from infrared to X-ray wavelengths. In this technique, a relativistic electron beam is subjected to a periodic transverse magnetic field. The resultant periodic deflection of the electron beam causes stimulated emission of radiation from the electron beam. Tuning is accomplished by varying the electron beam energy or by varying the magnetic field strength. A drawback of this technique is the large size and weight of the hardware required for implementation.
U.S. Pat. No. 3,958,189 describes a tunable high power submillimeter radiation source which generates radiation by backscattering from an intense relativistic electron beam. Here the relativistic electron beam and microwave energy counter-propagate in a parallel magnetic field. The frequency of the microwave energy is tuned to be mismatched by a specific amount from the electron cyclotron frequency in the rest frame of the electrons at high beam current. As a result, electron bunching occurs which acts to scatter the microwave energy to generate submillimeter radiation. Tuning is accomplished by tuning the microwave frequency or by changing the energy of the electron beam. The necessity of generating a cold relativistic electron beam is the major handicap of this technique.
The techniques exemplified by the above-described U.S. patents are all of a general type known as free electron lasers which are based on relativistic electron beams. In free electron lasers, a portion of the kinetic energy present in a relativistic beam of free, or unbound electrons, is converted into the desired wave energy via the interaction between the electron beam and a rippled magnetic field and, possibly, other forms of radiation. This is to be distinguished from conventional lasers wherein radiation is produced by the stimulated emission from an inverted population of bound electrons in a lasing atomic/molecular medium.
One of the major disadvantages of known free electron lasers is the requirement that the electron beam be of a relativistic energy level. Additionally, these devices require large magnetic fields frequenctly having complicated field profiles. These requirements increase the size, complexity, and cost of such devices and greatly complicate the tuning of these devices.
Much current research effort is being expended in the area of free electron lasers in order to improve their efficiency, size, economics, and recycling. Of particular interest is the development of free electron lasers for use in the 8 to 10 .mu.m wavelength "atmosphere window" range wherein reduced atmospheric absorption is exhibited. This spectral range is highly desirable for aircraft countermeasures applications.
The present Invention is directed to a novel rapidly tunable radiation generator for use in the 8 to 10 .mu.m wavelength range which does not require a relativistic electron beam for generation. The generator combines the principles of the free electron laser with bound electron lasers and thus is termed a synergistic quasi-free electron laser.